Rice blast caused by rice blast fungi (pyricularia oryzae, Magnaporthe grisea) is recognized in most countries where rice is cultivated. In particular, in regions having climates of high temperature and high humidity (e.g., Japan), rice blast is one of the most serious diseases in agricultural industry. For high yield rice cultivation, prevention of and disinfestation for rice blast are essential. Recently, as an alternative to agents with treatment effects, box-treatment agents having preventive effects are used for reducing the labor of farmers in prevention and disinfestation regarding rice blast fungi. Examples of such agents include scytalone dehydratase (hereinafter, simply referred to as “SCDH”) inhibitors as typified by carpropamid ((1RS,3SR)-2,2-dichloro-N-((R)-1-(4-chlorophenyl)ethyl)-1-ethyl-3-methylcyclopropanecarboxamide)) (Kurahashi et al., J. Pestic. Sci, 23, 22-28, 1998; Motoyama et al., J. Pestic. Sci, 23, 58-61, 1998). SCDH is an enzyme that catalyzes the dehydration reaction from scytalone to 1,3,8-trihydroxynaphtalene (hereinafter, simply referred to as “1,3,8-THN”) in melanin biosynthesis pathways.
When a rice blast fungus ruptures and invades a cuticular membrane of a rice leaf surface, the concentration of glycerol in the appressorium, an infection-specific organ, increases up to 80 atm. In order to enclose the glycerol within the appressorium, the melanin layer of the cell wall is essential (Kamakura et al., KASEAA, 39, 340-347, 2001). Inhibition of melanin biosynthesis prevents formation of the appressorium. Thus, SCDH inhibitors do not have a direct fungicidal action, but rather are non-fungicidal agents that exhibit prevention and disinfestation activities by suppressing pathogenicity.
An SCDH gene from a filamentous fungus was first elucidated with Pyricularia oryzae. The nucleotide sequence of this gene was not available to the public and only the three-dimensional structure of the SCDH protein was reported (Landquist et al., Structure, 2, 937-944, 1994). Thereafter, an SCDH gene from Colletorichum lagenarium (Kubo et al., Appl. Environment. Microbiol, 62, 4340-4344, 1996; Accession no. D86079), followed by SCDH genes from Aspergillus fumigatus (Tsai et al., Mol. Microbiol, 26, 175-183, 1997; Accession no. U95042), Pyricularia oryzae (Motoyama et al., Biosci. Biotech. Biochem, 62, 564-566, 1998; Accession no. AB004741) and Ophiostoma floccosum (Wang et al., Accession no. AF316575) were reported. A three-dimensional structure of an SCDH protein bound to carpropamid has also been reported (Nakasako et al., Biochemistry, 37, 9931-9939, 1998; Wawrzak et al., Proteins: Struct. Func. Genet, 35, 425-439, 1999).